We don't need is_recommended any longer - is_promoted has the same information as recommended addons are now a subset of promoted addons. Let's remove it from the api and add a shim for the property in api/v4 (it could exist in v3 too, but it was only added after v4 was launched so is undocumented in v3). Or maybe just drop entirely once addons-frontend has switched over?

Dropping the data from the ES index depends on us replicating the the recommended filter and sort, or at least rewriting them to rely on promoted data in the ES index instead.

--edited after #7910 dropped the model property